In the past, for electronic packaging, it was common to form a housing having internal integral panels with electronic substrates attached to both sides of the panels. In order to route connections between the two substrates, it was necessary to fire-in glass feed throughs. The glass feed throughs required that glass beads having through openings therein would have metal pins inserted in the glass beads. Openings for the route connections would be machined into the panels, the glass beads with the pins would be placed and held in the panel openings, and the entire housing fired in an oven to melt the glass to hold the pins in their proper positions. The housing is then gold plated (including the pins). The substrates would then be secured to both sides of the panel and wires, generally of gold, would be bonded to the substrates and then to the ends of the pins to complete the double-sided module. Another option is a soldered-in feed through.
The above technique had many problems. Sometimes, during the oven firing and cooling process, the glass would develop cracks, chips, holes, or bubbles, which would mean the entire housing would have to be rejected. Even one such flaw would be the basis for rejecting the entire housing. Other times, the glass would climb or melt out of the opening in the panel and solidify on the panel surface. This would prevent the substrate from sitting flat on the panel. By not sitting flat on the panel, the substrate could potentially fail electronically or be destroyed from the mechanical stress of acceleration testing, so housings would be rejected for this reason.
Further, the pin could float from one side to the other during the firing process and move so far off center or be angled such that the substrates could be short-circuited together. Even further, since each feed through had four connection points, two to the substrates and two to the pin, these added additional areas of potential failure.
Also, the pins could be improperly plated, which would be another basis for rejecting the housing. The firing temperature for the glass is above the melting point of the gold. This means the pins must be individually wire-wrapped and electro-chemically gold plated after assembly and firing in an extremely labor intensive process.
In addition, on the design side, the fired-in glass feed through created a limitation on the packing density of the number of feed throughs possible within a given panel space. This is because a certain minimum diameter opening is required to fit in the pin and glass bead combination.
Finally, manufacturing was very costly because special graphite fixtures were required due to the high temperatures of firing and because a great deal of manual assembly was required.
Although rejecting a housing was expensive, no defects could be tolerated because of the critical applications in which the double-sided modules were used. While other alternatives were sought, as the use of metal pins for feed throughs has been an industry standard for over twenty years because no better approach has previously been discovered.